The present Invention is in the field of waterway rehabilitation and pertains more particularly to simulated large woody debris and methods for manufacturing same.
For many years it had been a practice to remove debris, such as logs or brush piles from streams. In many cases this was ostensibly to enhance appearance and visual esthetics or improve hydraulic conveyance. Unfortunately, the ultimate result was more usually detrimental to the stream and its faunal inhabitants, both insect and vertebrate. Natural pools were lost and streams tended to fill and braid. The result was frequent loss of habitat and greatly changed hydraulic characteristics.
Stream habitat restoration and management reportedly began in the United States in Michigan in the 1920""s. Early structures were designed and installed with little understanding of the ecology of stream systems. Most structures were approached as civil works. It wasn""t until the 70s that aquatic biologists, fisheries scientists and hydrologists began to more fully understand the role of woody debris in stream ecosystems and the associated limitations of artificial in-stream structures. By the early 1980""s the role of large woody debris associated with old-growth forests became the focal point for intense study. Today, stream restoration and improvement projects almost universally include large woody debris elements. A large body of literature has developed relative to the importance of large woody debris and its most efficient placement in streams.
Large woody debris is not uniformly defined among stream ecologists but historically it has usually been considered as fallen trees and logs with major diameters of about 20 inches or greater. In northwestern United States the term is almost universally used in reference to the fallen large trees found in virgin old growth forests. These trees frequently have been uprooted from stream banks by undercutting and retain large root wads that help to anchor and stabilize them.
While organic debris of all sizes is generally recognized as important for maintaining the biotic and abiotic functions of stream channels, large woody debris is critical. Large woody debris has a major influence on channel form, sediment transport and deposit patterns. The quantity of large woody debris in a channel system is highly correlated with the number of pools. Many experts state that large woody debris of proportions associated with old growth forests will be lacking for at least the next hundred years. It has been suggested that loggers and landowners not log trees that are within 150 feet of a stream so that they will eventually fall into that stream thereby providing large woody debris to that stream. Legislation has been suggested in this area also.
The demand for large woody debris to use in stream restoration has resulted in scarcity and rapidly increasing costs. The dominant source of large woody debris is from flood debris clean-up, land clearing, and road building projects. Contractors now recognize that waste wood and stumps have a market value and the cost has risen accordingly.
Modification of stream channels to improve habitat dates from the early 1900s. The most common methods to improve or manage instream habitat and/or hydrology include current deflectors, low-head dams, weirs, planted vegetation and boulder placement. Placement of large woody debris is a relatively recent advancement, driven by the recognition that it provides many previously under-appreciated functional values. Because of the scarcity of large woody debris and high expense for placement, logs are placed in the stream and anchored to the bedrock, anchor rocks, or stream banks with steel cables. Cabling is necessary to prevent the relatively light logs from being washed downstream where they might collect in logjams or against public or private structures. This approach has a number of limitations. First, the logs used are otherwise merchantable and have significant value. Second, Logs tend to rot and are therefore not a long-term solution. Third, logs are buoyant with a specific gravity of around 0.44 to 0.55. With moderate to high water flow in winter a large percentage placed woody debris logs are washed downstream into private or public structures with a potential to cause considerable damage. Forth, in the event the logs washed downstream do not damage bridges or other private structures the will eventually end up on beaches or clogging up lakes where they can become very hazardous to boaters and the like.
What is clearly needed is an alternative to natural woody debris that is heavy enough to stay in place, cost effective, has the exact look and feel of natural woody debris, easily transportable by helicopter and conventional means, will last for hundreds of years.
In a preferred embodiment of the present invention a cast-concrete simulated log having a longitudinal axis and an outside surface simulating a natural log appearance is provided, the log also having at least one triangular rebar reinforcement embedded in the concrete in a plane at substantially a right angle to the longitudinal axis, with corners of the triangular rebar exposed at the outside surface, such that the exposed corners of the rebar triangle are available as connection points for attachment to other structures or for lifting and manipulating the log.
In some embodiments the log has two or more embedded rebar triangles spaced along the length of the log, and also in some embodiments the concrete is colored to simulate the color of a natural log.
In another aspect of the invention large-woody-debris assembly is provided, comprising two or more cast-concrete simulated logs, each having a longitudinal axis and an outside surface simulating a natural log appearance, and each having at least one triangular rebar reinforcement embedded in the concrete in a plane at substantially a right angle to the longitudinal axis, with corners of the triangular rebar exposed at the outside surface, the two or more simulated logs connected to one another between corners of the rebar triangles on each log. There may be two or more triangular rebar reinforcements embedded at different position along the longitudinal axis of each simulated log, and in some embodiments there are three simulated logs connected in a close triangular arrangement between exposed comers of the rebar triangles in each simulated log. The concrete may be colored to simulate the color of a natural log.
In yet another aspect a mold for forming a cast-concrete simulated log is provided, comprising a first mold element having a length and a semicircular cross-section, a second mold element having substantially the same length and semicircular cross section as the first mold element, the second mold element hinged to the first such that the two mold elements may be closed to form a circular mold having an inside surface, and a polymer lining adherent to the inside surface of each mold element, the polymer lining having itself an inside surface formed as a female replica of a surface of a natural log.
In some embodiments the polymer lining is created by suspending a natural log within a closed mold, the log substantially evenly spaced apart from the inside surface of the mold elements, creating thereby an annular cavity between the log and the mold elements, pouring a polymer material into the annular cavity, curing the polymer, and removing the log.
In yet another aspect of the invention a method for creating a cast-concrete simulated log is provided, comprising the steps of (a) creating a mold with a length and a substantially circular cross-section, the mold having an inside surface simulating the appearance of a natural log; (b) suspending at least one triangular rebar in a plane substantially at a right angle to the length; and (c) casting concrete in the mold such that, when opened, the cast-concrete simulated log has an outer surface simulating a natural log, and all three corners of the triangular rebar are exposed at the outer surface of the simulated log.
In some embodiments of the two or more triangular rebars are suspended at different places along the length of the mold, to create two or more reinforcements with exposed corners suitable for connecting to other structures and for picking up and placing the simulated log. There may also be a step for coloring the concrete to more closely simulate the appearance of a natural log.
In still another aspect of the invention a method for forming a large-woody-debris assembly is provided, comprising (a) forming two or more cast-concrete simulated logs, each having a longitudinal axis and an outside surface simulating a natural log appearance, and each having at least one triangular rebar reinforcement embedded in the concrete in a plane at substantially a right angle to the longitudinal axis, with corners of the triangular rebar exposed at the outside surface; and (b) connecting the two or more simulated logs to one another between corners of the rebar triangles on each log. In some preferred embodiments three logs are formed, each having two or more triangular rebar reinforcements spaced along the longitudinal axis, and the three logs are assembled into a close triangular grouping by connection between the rebar corners.
In embodiments of the invention taught in enabling detail below, for the first time simulated large woody debris is provided in an economical manner, and in a durable form that may be expected to remain in place for a long time after placement in a stream bed. Further, means for easy manipulation and placement is provided.